Tutorials

Before you commit the money to buy (or the time to build) a pulse-measurement device, we recommend that you take a look at these tutorials, which will give you an idea of what's involved in measuring an ultrashort laser pulse.

 

Invented in the 1960s, intensity autocorrelation was the first pulse-measurement technique. It uses the  pulse to measure itself, splitting the it in two, variably delaying one with respect to the other, and  overlapping them in some rapidly responding nonlinear-optical medium.  Unfortunately, it yields only a blurry black and white image of the pulse intensity.  Nevertheless, it's important to understand it because FROG and GRENOUILLE are simply spectrally resolved autocorrelations--a seemingly minor variation, but, it turns out, a major improvement.

Interferometric Autocorrelation

 

Invented in the 1980s, interferometric autocorrelation was the second technique used to measure ultrashort laser pulses. Like intensity autocorrelation, it uses a laser pulse to measure itself, but it also interferes the light generated in the nonlinear-optical medium with light generated by the individual beams.  It yields some phase information, but not enough to determine the pulse intensity and phase.  This tutorial examines the characteristics and shortcomings of this also obsolete method.  Basically, it yields information essentially equivalent to the intensity autocorrelation and the spectrum.

FROG

 

Frequency-resolved-optical-gating (FROG) is the only reliable technique for completely characterizing an ultrashort laser pulse in time. Since its introduction in 1991, FROG has evolved into a marvelously general and powerful technique for measuring ultrashort laser pulses. Instead of measuring an autocorrelator signal energy vs. delay, which yields a simple autocorrelation, FROG involves measuring the signal spectrum vs. delay. This extra dimension is the key to measuring pulses completely and without the need for assumptions. This tutorial introduces the FROG technique and its capabilities.

Old-Fashioned Scanning FROG

 

The original FROG device involved scanning the delay between two pulses.  This meant clumsy translation stages, moving parts, and the need to re-align the device every time it was bumped, moved, or even allowed to remain in place for more than a few hours or days.  Worse, it tends to be sold as an all-purpose measurement device, but the user spend much money and must take it apart and replace all optics to extend to another wavelength range.  You might as well build your own FROG to begin with.  Some companies still sell this device, but it's almost as obsolete as the autocorrelator.

 

A simplified elegant FROG device, GRENOUILLE combines the full-information pulse measurement capability of FROG with extreme experimental simplicity. GRENOUILLE operates alignment-free, requires only a few simple optical elements, and naturally operates single-shot. This tutorial introduces GRENOUILLE, teaches its operating principles, and shows why it is a superior technique for measuring ultrashort laser pulses. It also describes its unique ability to measure the elusive spatio-temporal distortions—like spatial chirp and pulse-front tilt—deleterious distortions common to ultrafast lasers.

 

Ultrashort laser pulses lead difficult lives. They’re routinely dispersed, stretched, amplified, and eventually compressed to, we hope, their shortest possible width.  Whether the source is an oscillator, a regen, or a high-power amplifier, ultrashort pulses undergo massive manipulations in order to become so short.  But at what price?  Possible spatio-temporal distortions, like angular dispersion and pulse-front tilt.  This tutorial present these and other typically encountered pulse distortions and the importance of accurate observation and characterization.

 

While all optical devices have wavelength-dependent specs to some extent, GRENOUILLE does so a bit more.  This tutorial explains why and what you need to know about this effect to use a GRENOUILLE at different wavelengths.  To summarize, for longer wavelengths, GRENOUILLE can measure shorter pulses.  This is because GRENOUILLE takes advantage of dispersion in nonlinear-optical crystals for its spectral resolution, and crystal dispersion decreases as the light wavelength moves from the visible to the infrared.

Spatial Profile Effects in GRENOUILLE

 

Initially, due to its single-shot beam geometry, which uses the transverse spatial coordinate for delay, it seemed that GRENOUILLE could only measure a pulse with a perfect spatial profile; otherwise an inaccurate measurement of the pulse would result.  But GRENOUILLE's use of a thick SHG crystal actually is an interesting feature never before  encountered in pulse measurement.  And more interestingly, this feature actually almost completely removes the beam-profile requirement. The power of the FROG algorithm also helps to make this happen.  Read this tutorial to see why.

 

Pulse compression is not only an alignment-intensive task, but it can also introduce problematic spatio-temporal distortions into your pulse, which limit the intensity your pulse can achieve, especially at a focus.  But Swamp Optics' award-winning single-prism pulse compressor solves all of these problems--it is physically incapable of distorting your pulse!  In addition, it is a compact, alignment-free, easy-to-work-with device that is also inexpensive—about half the price of other commercially available pulse compressors. Simplicity at its best!

 

Researchers in the 1960s often mistook a narrow spike in their autocorrelations, called the "coherent artifact," for the measure of their pulse length, but its width actually only yields the shortest component of the pulse, not the true pulse length. This misinterpretation continues today, as most new measurement techniques yield only the coherent artifact, and not the pulse, but this is not well known.  This astounding case of self-deception (this is the second time it has happened!) continues, but you can help educate the community to help improve ultrafast measurements. Read our tutorial.

Trebino group presentations

Trebino group publications

Publications:  Books

Frequency-Resolved Optical Gating: The Measurement of Ultrashort Laser Pulses, by Swamp Optics' own Rick Trebino

The only book ever written on the subject of pulse measurement!

 

Available for purchase from Amazon.com and any technical book store.

Publications:  Ultrafast Optics Textbook!

Ultrashort Light Pulses and Their Measurement, also by Swamp Optics' own Rick Trebino.  Coming in 2016.

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